Study of the expression of concurrent operation within programming languages has yielded a data-driven form of program representation known as data flow. The development of the data-flow form of representation was accompanied by the development of a processor designed to exploit the parallelism exposed by the data-flow form in the execution of arithmetic and conditional program constructs. The architectures of two such processors are described in related applications Ser. No. 456,488, now U.S. Pat. No. 3,962,706, and Ser. No. 605,932 which are incorporated into the present specification by reference.